The invention relates to an antenna stabilization system, and, more particularly, to an antenna support structure oriented by linear actuators responsive to linear position transducers of a passive stabilization system serving as a vertical reference and control network therefor.
It is generally the practice when utilizing communication antennas on land or water to mount the antenna wherein its radiation pattern remains essentially in a fixed reference frame. It is similarly common practice to incorporate gimbaled supporting structure to facilitate angulation and orientation thereof. This conventional practice of mounting has many applications such as satellite tracking for communication and navigational purposes. Frequently such applications also require antenna stabilization systems which compensate for certain movements such as pitch and roll motions of the supporting platform in order to maintain the preselected frame of reference of the antenna. Consequently, antenna support apparatus has been developed for compensating for such movement as it occurs and for detecting the relative magnitude thereof for integrating such information into compensational error signals which cause the antenna supporting structure to maintain the preselected frame of reference. Similarly, apparatus has been developed for generating the coordinate reference base for sensing relative motion. In particular, zero error reference frames have been developed for such systems wherein the reference frame is comprised of inertial coordinates from which compensation signals may be generated to maintain the preselected coordinate frame with zero deviation error. The methods and apparatus providing this reference system for antenna position control have recently found widespread application in offshore energy exploration operations.
The utilization of satellite communication terminals aboard offshore drilling and production platforms offers substantial operational and cost advantages to the oil and gas industry. Efficient utilization of such terminals, however, is largely dependent upon the ability to keep a relatively large antenna precisely pointed at a geostationary satellite during all types of motion disturbances. Such disturbances take the form of roll, pitch and yaw motions as experienced on semi-submersible platforms and, to a larger extent, on drilling ships. Because satellite system requirements often dictate beam stability better than 0.8.degree., other motion disturbances such as structural deflections due to wind, ice and wave action on both jack-up rigs and fixed production rigs are also primary considerations with regard to the antenna position.
Prior art apparatus for stabilizing antenna systems and other commercial and military hardware has included the aforementioned compensational error signal variety adapted for offshore applications. In an effort to maximize efficiency and response time, systems may also incorporate the pendulous properties of the antenna itself to assist in maintaining the preselected reference frame. One such system, incorporating both methods, is described and claimed in British Pat. No. 890,264, entitled "Rotatable Antenna Assembly", the complete specification of which was published on Feb. 28, 1962, having previously been filed in the U.S. on Feb. 2, 1959. The system set forth therein is characterized by dual axis rotation sensing means coupled to roll and pitch detecting gyroscopes and incorporated into a network of torquers adapted for first and second generation rotational actuation in response to the aforementioned sensor and gyroscopic outputs. In this manner the mass of the antenna itself may be utilized in compensating for undulations of the supporting structure while the reference plane error detected is ostensibly driven toward zero.
Certain other prior art apparatus has included strapped down level and rate sensors for detecting yaw, pitch and roll of an antenna mounted on a gimbaled plane. The signals from the sensors are integrated to actuate roll and pitch axis sensors adapted for negating error signals and maintaining the gimbal level in preselected inertial coordinates comprising the frame of reference. The actuators for such an apparatus are generally comprised of servo motors operating therefrom. The servo drive packages are conventionally mounted within the antenna pedestal base beneath the sensor package in order to orient the antenna and controlling sensors which are strapped to the same reference plane therewith.
The aforesaid apparatus has been shown to be effective in meeting the prior art demands of accuracy and cost effectiveness. However, the need for more reliable stabilization systems and more cost effectiveness therewith in the search for offshore energy reserves has fostered the development of improved antenna stabilization systems such as those exhibiting first generation error compensation and shorter response time. As disclosed above, it may be seen that many conventional prior art approaches incorporate error detection and compensation systems which include compensatory actuation for maintaining zero error detection in the sensor package and in the antenna structurally tied thereto. For example, the torquer units of the British patent above-referenced are utilized to correct rotational deviations in both the sensor package and actuation network. Such efforts, even when incorporating the pendulous effect of the antenna, involve relatively complex network feedbacks for driving the error signal to zero. Similarly, the strapped down sensor packages of related prior art approaches primarily drive the antenna and sensor platform toward a zero error reference. The necessity for complex feedback control is aggravated by the need of strictly linear actuation. Inherent component errors of such systems are similarly amplified in recycling feedback networks generating over compensation when roll and/or pitch amplitudes approach higher levels of the type convantionally encountered in high seas.
It would be an advantage therefore to overcome the problems of prior art apparatus by providing an improved stabilization system for antenna mounts, optical mounts and/or military artillery mounts incorporating true linear compensational actuation without deriving error signals from a zero error reference frame. The stabilization system of the present invention is especially adapted for true linear actuation in direct response to first generation error sensing. Moreover, error compensation actuation is confined to the apparatus supporting frame rather than including a sensor package because the only true reference plane is inertially stabilized and requires no error correction. Since all primary movement is detected relative to an inertially stabilized reference, all motion can be broken down into a combination of linear movements comprising the coordinate frame of both the antenna-actuator combination and reference plane-sensor combination. In this manner linear position transducers may be utilized where heretofore impracticable and direct antenna compensation signals generated in a control network functioning as an analog computer rather than a feedback network. Inherent component errors may thus be reduced and response time shortened to achieve accuracy and control heretofore unfeasible with compatible commercial cost levels.